You're running on a not supported OS, the OS check failed
+ remote_os_domain=versions.pi-hole.net
+ valid_os=false
+ valid_version=false
+ display_warning=true
++ grep PRETTY_NAME
++ cat /etc/os-release
++ cut -d = -f2-
++ tr -d '"'
+ detected_os_pretty='Open Source Media Center'
+ detected_os=Open
++ grep VERSION_ID
++ cut -d = -f2-
++ cat /etc/os-release
++ tr -d '"'
+ detected_version=2020.06-1
+ IFS=' '
+ read -r -a supportedOS
++ dig +short -t txt versions.pi-hole.net @ns1.pi-hole.net
++ tr -d '"'
You can skip the check with
curl -sSL https://install.pi-hole.net | PIHOLE_SKIP_OS_CHECK=true sudo -E bash